U.S. patent application number 11/576050 was filed with the patent office on 2009-07-16 for rare earth sintered magnet.
This patent application is currently assigned to TDK CORPORATION. Invention is credited to Yasushi Enokido, Chikara Ishizaka, Gouichi Nishizawa.
Application Number | 20090178735 11/576050 |
Document ID | / |
Family ID | 36407155 |
Filed Date | 2009-07-16 |
United States Patent
Application |
20090178735 |
Kind Code |
A1 |
Enokido; Yasushi ; et
al. |
July 16, 2009 |
RARE EARTH SINTERED MAGNET
Abstract
The present invention provides a technique to improve an
adhesion strength between a magnet main body and a protective film.
The rare earth sintered magnet of the present invention comprises a
magnet main body of a sintered body containing a rare earth element
and a protective film formed on the magnet main body, wherein the
ratio of a 10-point average surface roughness Rz of the magnet main
body on which the protective film is formed to a mean grain size
D50 in the magnet main body (Rz/D50 ratio) is kept in a range from
0.20 to 10.00, inclusive. This gives the rare earth sintered magnet
which is coated with the protective film having a high adhesion
strength of 100 N/m or more and exhibits high corrosion
resistance.
Inventors: |
Enokido; Yasushi; ( Tokyo,
JP) ; Nishizawa; Gouichi; ( Tokyo, JP) ;
Ishizaka; Chikara; ( Tokyo, JP) |
Correspondence
Address: |
PEARNE & GORDON LLP
1801 EAST 9TH STREET, SUITE 1200
CLEVELAND
OH
44114-3108
US
|
Assignee: |
TDK CORPORATION
Chuo-ku, Tokyo
JP
|
Family ID: |
36407155 |
Appl. No.: |
11/576050 |
Filed: |
November 17, 2005 |
PCT Filed: |
November 17, 2005 |
PCT NO: |
PCT/JP05/21083 |
371 Date: |
May 19, 2007 |
Current U.S.
Class: |
148/302 |
Current CPC
Class: |
H01F 1/0577 20130101;
H01F 41/26 20130101; C22C 38/005 20130101; H01F 41/026 20130101;
Y10S 428/90 20130101; H01F 7/0221 20130101 |
Class at
Publication: |
148/302 |
International
Class: |
H01F 1/01 20060101
H01F001/01 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 17, 2004 |
JP |
2004-333479 |
Claims
1. A rare earth sintered magnet comprising a magnet main body of a
sintered body containing a rare earth element and a protective film
formed on the magnet main body, characterized in that the ratio of
a 10-point average surface roughness Rz of the magnet main body on
which the protective film is formed to a mean grain size D50 in the
magnet main body (Rz/D50 ratio) is in a range from 0.20 to 0.81
inclusive, the D50 is 2.0 to 15.0 .mu.m, and the Rz is 1.5 to 11
.mu.m, the rare earth sintered magnet is an R-T-B system sintered
magnet, and the protective film has an adhesion strength of 100 N/m
or more.
2-9. (canceled)
10. The rare earth sintered magnet according to claim 1,
characterized in that the protective film is formed by plating.
11. The rare earth sintered magnet according to claim 10,
characterized in that the protective film is formed by electrolytic
plating.
12-15. (canceled)
16. The rare earth sintered magnet according to claim 10,
characterized in that the protective film comprises Ni.
17. The rare earth sintered magnet according to claim 11,
characterized in that the protective film comprises Ni.
18. The rare earth sintered magnet according to claim 1, 10, 11, 16
or 17, characterized in that the R-T-B system sintered magnet
comprises 25 to 37 wt % of R, 0.5 to 4.5 wt % of B and 0.02 to 0.5
wt % of one or both of Al and Cu, the balance being substantially
T, wherein a is one or more rare earth elements, T is Fe or Fe and
Co, and B is boron.
19. The rare earth sintered magnet according to claim 1, 10, 11, 16
or 17, characterized in that the D50 is 2.5 to 8.0 .mu.m.
20. The rare earth sintered magnet according to claim 18,
characterized in that the D50 is 2.5 to 8.0 .mu.m.
Description
TECHNICAL FIELD
[0001] The present invention relates to a rare earth sintered
magnet, represented by a Nd--Fe--B system permanent magnet, in
particular a rare earth sintered magnet coated with a protective
film.
BACKGROUND ART
[0002] Rare earth permanent magnets have been widely commercialized
for their excellent magnetic properties. However, they contain a
rare earth element and iron as main constituents, which are easily
oxidized, and have relatively low corrosion resistance, so that
their magnetic properties are deteriorated by oxidation. Therefore,
their magnet main bodies are coated with a various protective
film.
[0003] Patent Document 1, for example, discloses a permanent magnet
comprising a magnet main body with surface irregularities, coated
with an electroplated layer and an electrolessly plated layer.
Patent Document 1 discusses that adhesion between the magnet main
body and the plated layers formed thereon is improved by limiting
its R.sub.max value (index of surface roughness, defined by JIS
B-0610, according to the cited document 1) to a range from 3 to 50
.mu.m.
[0004] Patent Document 2 discloses that a magnet main body can be
coated with a protective film of high peel resistance, when it has
a 10-point average surface roughness of 5 to 100 .mu.m, defined by
JIS B-0610, on the grounds that adhesion is insufficient at a
roughness of 5 .mu.m or less, and no improvement in adhesion is
observed and product value is conversely deteriorated at 100 .mu.m
or more.
Patent Document 1: Japanese Patent Laid-Open No. 2-185004 (Claims,
page 5)
Patent Document 2: Japanese Patent Laid-Open No. 7-66032
(Claims)
DISCLOSURE OF THE INVENTION
[0005] Patent Documents 1 and 2 propose that an adhesion strength
between a magnet main body and a protective film can be improved by
controlling a surface roughness of the uncoated magnet main
body.
[0006] However, a rare earth permanent magnet is demanded to be
coated with a protective film of higher adhesion strength, when it
is pressed into a case for use, because it is subjected to a stress
working so as to peel the protective film off from the magnet while
it is being pressed into the case.
[0007] The present invention is developed to solve these technical
problems. It is an object to provide a technique to improve an
adhesion strength between the magnet main body and the protective
film.
[0008] The present inventors have focused on the fact that in terms
of a mechanism involved in adhesion of a protective film, bonding
between the protective film and a magnet main body is based on
physical bonding rather than chemical one, and that the adhesion
strength of the protective film is determined not only by the
surface roughness of the magnet main body on which it is formed but
also by the size of grains on the interface between the protective
film and the magnet main body. They have found that a rare earth
sintered magnet with a protective film adhering thereto at a high
adhesion strength and with a high corrosion resistance can be
realized by controlling the ratio of the 10-point average surface
roughness to the mean grain size of the sintered body in the magnet
main body in a predetermined range.
[0009] The present invention provides a rare earth sintered magnet
comprising a magnet main body of a sintered body containing a rare
earth element and a protective film formed on the surface of the
magnet main body, characterized in that the ratio of the 10-point
average surface roughness (hereinafter referred to as "10-point
average roughness Rz" or "Rz") of the magnet main body, on which
the protective film is formed, to the mean grain size (hereinafter
referred to as "grain size D50" or "D50") in the magnet main body
is kept in a range from 0.20 to 10.00 inclusive, the ratio being
hereinafter referred to as "Rz/D50." The grain size D50 is
determined by image-processing of area of each grain present in the
vicinity of interface between the magnet main body and the
protective film, more specifically within a depth of about 100
.mu.m from the interface. The procedure for determining the grain
size D50 for the present invention will be described later in more
detail in Examples, together with the procedure for determining the
10-point average roughness Rz.
[0010] The protective film is required to be sufficiently dense and
defect-free to prevent permeation of oxygen, and, at the same time,
to adhere to the rare earth sintered magnet surface at a high
adhesion strength. The present invention, characterized by the
Rz/D50 ratio within the above range, can have a high adhesion
strength of 100 N/m or more, determined in accordance with JIS
H-8504.
[0011] Keeping an Rz/D50 ratio in a range from 0.20 to 6.00,
inclusive, can realize a rare earth sintered magnet coated with a
protective film adhering thereto at a high adhesion strength and
also high in corrosion resistance.
[0012] The type of protective film formed on the magnet main body
is not limited, but preferably is formed by plating.
[0013] The present invention can provide a rare earth sintered
magnet coated with a protective film strongly adhering to the
surface without deteriorating magnet corrosion resistance.
BRIEF DESCRIPTION OF THE DRAWING
[0014] FIG. 1 is a graph illustrating the relationship between
Rz/D50 ratio and adhesion strength.
BEST MODE FOR CARRYING OUT THE INVENTION
[0015] The present invention is described in more detail below.
<Rare Earth Sintered Magnet>
[0016] First, the rare earth sintered magnet which the present
invention is directed to is described.
[0017] The present invention is preferably applied to an R-T-B
system sintered magnet, wherein R is one or more of rare earth
elements, T is Fe or Fe and Co, and B is boron. It is because the
R-T-B system sintered magnet is insufficient in corrosion
resistance and will have to be coated with a protective film.
[0018] The R-T-B system sintered magnet contains a rare earth
element (R) at 25 to 37 wt %, wherein R for the present invention
has a concept which includes Y, R being one or more selected from
among Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.
At an R content below 25 wt %, the R-T-B system sintered magnet may
have a greatly deteriorated coercive force, because the
R.sub.2T.sub.14B phase as the main phase in the magnet is formed
insufficiently, and .alpha.-Fe or the like showing soft magnetism
separates out. At above 37 wt %, on the other hand, the magnet may
have a deteriorated residual magnetic flux density, because of
decreased volume ratio of the R.sub.2T.sub.14B phase as the main
phase in the magnet. At the same time, R will react with oxygen to
increase oxygen content in the magnet, which can lead to
deteriorated coercive force because of decreased content of the
R-rich phase, which effectively creates coercive force. The R
content is set at 25 to 37 wt %. It is preferably 28 to 35 wt %,
more preferably 29 to 33 wt %.
[0019] Moreover, the R-T-B system sintered magnet to which the
present invention is applied contains boron (B) at 0.5 to 4.5 wt %.
At a B content below 0.5 wt %, the magnet may not have a sufficient
coercive force. At above 4.5 wt %, on the other hand, the magnet
tends to have a decreased residual magnetic flux density.
Therefore, the upper limit of B content is set at 4.5 wt %. The B
content is preferably 0.5 to 1.5 wt %, more preferably 0.8 to 1.2
wt %.
[0020] The R-T-B system sintered magnet to which the present
invention is applied can contain Co at 3.0 wt % or less (excluding
0), preferably 0.1 to 2.0 wt %, more preferably 0.1 to 1.0 wt %,
still more preferably 0.3 to 0.7 wt %. Co forms a phase similar to
that formed by Fe, and is effective in increasing the Curie
temperature and in improving the corrosion resistance of the grain
boundaries.
[0021] The R-T-B system sintered magnet to which the present
invention is applied can contain Al and/or Cu in a range of 0.02 to
0.5 wt %. The sintered magnet can have improved coercive force,
corrosion resistance and temperature characteristics when it
contains Al and/or Cu in the above range. Al, when used, is
incorporated preferably at 0.03 to 0.3 wt %, more preferably 0.05
to wt %. Cu, when used, is incorporated preferably at wt % or less
(excluding 0), more preferably 0.03 to wt %.
[0022] The R-T-B system sintered magnet to which the present
invention is applied may further contain other elements, e.g., Zr,
Ti, Bi, Sn, Ga, Nb, Ta, Si, V, Ag, Ge and so forth. On the other
hand, it is preferable to reduce impurity elements, e.g., oxygen,
nitrogen, carbon and so forth, as far as possible. In particular,
oxygen content is preferably limited to 5000 ppm or less, more
preferably 3000 ppm or less, because oxygen has adverse effects on
magnetic properties. At a higher oxygen content, the sintered
magnet has an increased content of rare earth oxide phase(s) as
non-magnetic component(s) to deteriorate its magnetic
properties.
[0023] The present invention is preferably applied to an R-T-B
system sintered magnet, but applicable to other types of rare earth
sintered magnets, e.g., an R--Co system type.
[0024] The R--Co system sintered magnet contains R, one or more
elements selected from among Fe, Ni, Mn and Cr, and Co. Preferably
the magnet further contains Cu and one or more elements selected
from among Nb, Zr, Ta, Hf, Ti and V, and particularly preferably
contains Cu and one or more elements selected from among Nb, Zr,
Ta, Hf, Ti and V. Of these, in particular, intermetallic compound
of Sm and Co are present, preferably in the form of
Sm.sub.2Co.sub.17 as a main phase and a SmCo.sub.5 system auxiliary
phase in the grain boundaries. A specific composition may be
adequately selected depending on a production method adopted,
required magnetic properties and so forth. However, the magnet
preferably has a composition comprising R at 20 to 30%, in
particular about 22 to 28%, one or more of Fe, Ni, Mn and Cr at
about 1 to 35%, one or more of Nb, Zr, Ta, Hf, Ti and V at 0 to 6%,
in particular about 0.5 to 4%, and Cu at 0 to 10%, in particular
about 1 to 10%, the balance being of Co, all percentages by
weight.
[0025] The R-T-B system and R--Co system sintered magnets have been
described, which by no means limits application of the present
invention to other types of rare earth sintered magnets.
[0026] A rare earth sintered magnet tends to have a coercive force
increasing as its grain size (D50) decreases. Therefore, the grain
size (D50) is set at 2.0 to 15.0 .mu.m, preferably 10.0 .mu.m or
less, more preferably 2.5 to 8.0 .mu.m, still more preferably 2.5
to 6.0 .mu.m.
[0027] However, the grain size (D50) is 3.5 to 15.0 .mu.m, more
preferably 4.0 to 15.0 .mu.m viewed from securing a high residual
magnetic flux density.
<Protective Film>
[0028] The rare earth sintered magnet of the present invention
comprises a rare earth sintered magnet main body coated with a
protective film.
[0029] The protective film for the present invention is not
limited, but particularly preferably formed by electrolytic
plating. The electrolytic plating material may be selected from Ni,
Ni--P, Cu, Zn, Cr, Sn or Al, of which Ni is more preferable
although the other materials can be used. Moreover, these materials
may be simultaneously used to form a multi-layered film. A
protective film formed by electrolytic plating is a typical one for
the present invention, but may be formed by another method. For
example, any one of electroless plating, chemical treatment, e.g.,
chromate treatment, and coating with resin, or a combination
thereof is a practical method. Thickness of the protective film
should vary depending on size of the rare earth sintered magnet
main body, required corrosion resistance level and so forth.
However, it may be adequately set at 1 to 100 .mu.m, preferably 1
to 50 .mu.m, more preferably 1 to 20 .mu.m.
<Relationship Between Grain Size D50 and 10-Point Average
Roughnesses Rz>
[0030] Next, the relationship between the grain size D50 and the
10-point average roughness Rz as the most characteristic feature of
the present invention is described.
[0031] In the present invention, the ratio of the 10-point average
roughness Rz to the grain size D50 (Rz/D50 ratio) is set at 0.20 to
10.00, inclusive. At an Rz/D50 ratio below 0.20, the protective
film may have an insufficient adhesion strength. At an Rz/D50 ratio
above 10.00, on the other hand, the protective film may have
pinholes excessively, which deteriorate film corrosion resistance
because of accelerated corrosion on the magnet surface by water
permeating the pinholes, although the film has a good adhesion
strength. Moreover, the cost will be pushed up by additional extent
of surface roughening to have an Rz/D50 ratio beyond 10.00.
[0032] The rare earth sintered magnet coated with the protective
film can have an adhesion strength of 100 N/m or more while keeping
desired corrosion resistance when it has an Rz/D50 ratio at 0.20 to
10.00, inclusive.
[0033] It should be noted, however, that the effect of improving
adhesion strength of the protective film, brought by controlling
the Rz/D50 ratio, tends to be saturated and, at the same time, film
corrosion resistance starts to gradually decrease as the Rz/D50
ratio increases beyond 6.00. Therefore, the Rz/D50 ratio is
preferably set at 0.20 to 6.00, inclusive, more preferably 0.50 to
6.00, inclusive, to simultaneously keep high protective film
adhesion strength and corrosion resistance. An adhesion strength of
200 N/m or more can be secured at an Rz/D50 ratio of 2.00 to 6.00,
inclusive.
[0034] When importance is placed on corrosion resistance, the
Rz/D50 ratio is preferably set at 0.20 to 1.50, inclusive. Film
corrosion resistance can be kept very high at an Rz/D50 ratio in
the above range, preferably 0.50 to 1.00, inclusive, as discussed
later in Examples.
[0035] It is necessary to set the 10-point average roughness Rz
based on the grain size D50, as discussed above. However, corrosion
resistance tends to deteriorate as the 10-point average roughness
Rz increases beyond 40.0 .mu.m, and it is preferably set at 20.0
.mu.m or less, more preferably 1.5 to 20.0 .mu.m, still more
preferably 1.5 to 13.0 .mu.m, when the grain size D50 is about 2.0
to 15.0 .mu.m.
[0036] A magnet main body containing a rare earth element is
generally fragile. However, the magnet main body for the present
invention is coated with a protective film strongly adhering to the
surface, so that the magnet main body is resistant to external
forces.
[0037] A rare earth sintered magnet with a protective film may be
pressed into a given void in a member, and a stress resulting from
the pressing works to peel off the protective film. However, the
protective film for the rare earth sintered magnet of the present
invention has a sufficiently high adhesion strength of 100 N/m or
more to keep its adhesion strength under the stress.
<Production Method>
[0038] The method suitably adopted for producing the R-T-B system
sintered magnet of the present invention is described step by
step.
[0039] The starting alloy can be produced by a strip casting
process carried out under a vacuum or in an inert gas atmosphere,
preferably an Ar atmosphere, or other known dissolution processes.
A strip casting process sprays a starting metal melt, produced by
dissolving the metal in a non-oxidative atmosphere, e.g., Ar
atmosphere, onto a rotating roll. The melt quenched on the roll is
rapidly solidified into thin plates or thin pieces (flaky pieces).
The rapidly solidified alloy has a uniform structure having a grain
size of 1 to 50 .mu.m. The process is not limited to strip casting.
The starting alloy can be also produced by other dissolution
processes, e.g., radiofrequency induction dissolution. It can be
solidified on a water-cooled copper plate positioned at an angle to
prevent segregation of the molten alloy. Moreover, an alloy
produced by reduction diffusion process can be used as the starting
alloy.
[0040] For production of an R-T-B system sintered magnet, the
so-called mixing process in which an alloy mainly composed of the
R.sub.2T.sub.14B grains (low-R alloy) is mixed with an alloy
containing R at a higher content than the low-R alloy (high-R
alloy) can be adopted for the present invention.
[0041] The starting alloy is then passed to a crushing step, where
the low-R alloy and the high-R alloy, when a mixing process is
adopted, may be crushed simultaneously or individually. The
crushing step comprises a pulverizing step and a milling step.
First, the starting alloy is pulverized to a grain size of several
hundred microns preferably by a stamp mill, jaw crusher, Brown mill
or the like in an inert gas atmosphere. It is an effective
procedure to occlude the starting alloy with hydrogen for crushing
prior to the pulverizing step. Hydrogen is released from the alloy
later, because it works as an impurity for a rare earth sintered
magnet and is released to minimize its content in the magnet. The
alloy is kept at 200.degree. C. or higher, preferably 350.degree.
C. or higher for the occlusion. Occlusion time varies depending on
occlusion temperature adopted, starting alloy thickness and so
forth, but is at least 30 minutes or more, preferably 1 hour or
more. Hydrogen is released under a vacuum or in a flow of Ar gas.
However, it is not essential to adopt occlusion with hydrogen
followed by its release. When pulverizing using hydrogen is
employed, mechanical pulverizing may be saved.
[0042] The pulverizing step is followed by a milling step, for
which a jet mill is generally used to mill the pulverized powder of
several hundred microns to a mean particle size of 1.5 to 11.5
.mu.m, preferably 2.5 to 7 .mu.m, more preferably 3 to 7 .mu.m. A
jet mill ejects an inert gas of high pressure out of a fine nozzle
to generate a high-speed flow of the gas, by which the pulverized
coarser powder particles are accelerated and collided with each
other or with a target or container walls to divide them
finely.
[0043] When a mixing process is adopted, timing for mixing 2 types
of alloys is not limited. When the low-R and high-R alloys are
milled individually at the milling step, the finely milled low-R
and high-R alloy powders are mixed with each other in a nitrogen
atmosphere. The low-R/high-R alloy powders mixing ratio may be
about 80/20 to 97/3 by weight. The mixing ratio will be the same
when they are milled at the same time. The alloy powders may be
incorporated with an additive at about 0.01 to 0.3 wt % while being
finely milled to improve their lubricity and orientation for a
subsequent compacting step. The adequate additives include a fatty
acid and derivative thereof, e.g., a stearate or oleate, more
specifically zinc stearate, calcium stearate, aluminum stearate,
amide stearate, amide oleate or amide ethylenebisisostearate; or a
hydrocarbon, e.g., paraffin, naphthalene or the like.
[0044] The finely milled powders are then compacted into a
predetermined shape. This step is a magnetic field compacting in
which the powders are compacted in a magnetic field of a
predetermined intensity.
[0045] Compacting pressure in the compacting in a magnetic field
may be 0.3 to 3 tons/cm.sup.2 (30 to 300 MPa). The compacting
pressure may be constant throughout the step, or gradually
increasing or decreasing, or changed irregularly. Decreasing the
compacting pressure improves grain orientation, but gives a compact
of insufficient strength to cause handling troubles when it is
excessively low. In consideration of the above, the compacting
pressure is set at a level in the above range. The compacted body
produced by the compacting in a magnetic field generally has a
final relative density of 50 to 60%.
[0046] Intensity of the magnetic field to be applied may be about
12 to 20 kOe (960 to 1600 kA/m). The magnetic field to be applied
may be static, pulsed, or static and pulsed.
[0047] The resulting compacted body is then sintered under a vacuum
or in an inert gas atmosphere. It is necessary to adjust sintering
temperature in accordance with conditions, e.g., alloy composition,
crushing process adopted, mean particle size, particle size
distribution and so forth. However, the compacted body may be
sintered at 1000 to 1200.degree. C. for about 1 to 10 hours.
[0048] After the sintering, the resulting sintered body may be
subjected to an aging treatment. This is an important step for
controlling the coercive force of the sintered body. When the aging
treatment is carried out in two stages, it is effective to control
temperature at around 800.degree. C. and then at around 600.degree.
C. for a predetermined time. Heat treatment of the sintered body at
around 800.degree. C. increases its coercive force, particularly
effective when the mixing process is adopted. Heat treatment at
around 600.degree. C. greatly increases the coercive force of the
sintered body, and it is recommended to adopt this temperature
level when the aging treatment is carried out in one stage.
[0049] The treated sintered body is then cut into a predetermined
shape having predetermined dimensions.
[0050] After the cutting, the sintered body is treated to control
its surface roughness, before it is coated with a protective film.
This treatment processes the sintered body to provide
indentations/protrusions on the surface, thereby increasing
adhesive strength of a protective film to be formed thereon. A
desired surface roughness is set based on the grain size D50. More
specifically, the surface roughness of the sintered body is
controlled in such a way to keep the ratio of the 10-point average
roughness Rz to the grain size D50 (Rz/D50 ratio) at 0.20 to 10.00,
inclusive for the present invention, after the sintered body is
coated with a protective film.
[0051] The method for processing the sintered body surface is not
limited, but a mechanical processing is desired rather than a
chemical one so that the magnetic properties are not damaged. The
mechanical processing includes grinding with a grindstone, for
example.
[0052] The sintered body, after having desired surface conditions,
is coated with a protective film by a known method selected in
consideration of protective film type. When electrolytic plating is
adopted, the common procedure involves degreasing, washing with
water, etching (e.g., with nitric acid), washing with water,
electrolytic plating to form the film, washing with water and
drying. The sintered body surface may be cleaned by degreasing and
chemical etching with an acid.
[0053] Plating baths for electrolysis Ni plating useful for the
present invention include a Watt bath containing no nickel chloride
(solution is mainly composed of nickel sulfate and boric acid),
sulfamic acid bath, borofluoride bath and nickel boride bath. In
this case, the bath is preferably replenished with the nickel ion,
preferably in the form of nickel sulfate or nickel boride solution,
because of insufficient dissolution of the anode.
Example 1
[0054] A starting alloy was prepared by strip casting to have a
composition of Nd (26.5%)-Dy (5.9%)-Al (0.25%)-Co (0.5%)-Cu
(0.07%)-B (1.0%)-Fe (balance), all percentages by weight.
[0055] The alloy was then occluded with hydrogen at room
temperature and treated by hydrogen crushing at 600.degree. C. for
1 hour in an Ar atmosphere to release hydrogen.
[0056] The alloy treated by hydrogen crushing was incorporated with
a lubricant which worked to improve crushability and orientation in
the compacting step at 0.05 to 0.1%. A Nauter mixer may be used for
mixing the lubricant therewith for about 5 to 30 minutes. It was
then finely milled by a jet mill under varying conditions to have
milled powders of different particle size. Their sizes were
determined by a laser diffraction particle size distribution
analyzer. The results are given in Table 1.
[0057] The finely milled powders were compacted in a magnetic field
having an intensity of 15 kOe (1200 kA/m) under a pressure of 1.4
tons/cm.sup.2 (140 MPa).
[0058] The resulting compacted body was heated under a vacuum to
1080.degree. C., at which it was held for 4 hours for sintering. It
was then aging-treated in two stages, 800.degree. C. for 1 hour and
then 560.degree. C. for 1 hour, both in an Ar atmosphere.
[0059] Sintered bodies of different grain sizes were obtained by
sintering powders of different particle size under the same
conditions. The sintered bodies were then ground with a grind stone
to have different surface roughnesses. Each sintered body was then
electrolytic plated with Ni to a thickness of 10 .mu.m. The
Ni-plated film (i.e., protective film) was measured for adhesion
strength, in accordance with JIS H-8504.
[0060] The cross-section of the magnet coated with the Ni-plated
protective film was observed to determine its grain size D50 and
10-points average roughness Rz by the following procedures, and
based on the determined values the Rz/D50 ratio was calculated. The
results are given in Table 1. FIG. 1 presents the relationship
between the Rz/D50 ratio and the adhesion strength.
<Grain Size D50>
[0061] Photographs of the mirror-polished magnet cross-sections
were taken by a polarization microscopy. From the photographs, each
grain present in the vicinity of the interface between the magnet
main body and the plated film (within a depth of 100 .mu.m from the
interface and in a 100 by 100 .mu.m visual field) was
image-processed to determine its area, from which its diameter was
calculated by assuming that the grain was circular. The diameter,
determined from the two-dimensional FIGURE, was multiplied by 1.5
to find a diameter in the three-dimensional spherical FIGURE
corresponding to the circular FIGURE. This diameter of the sphere
was defined as the grain size D50.
<10-Point Average Roughness Rz>
[0062] Photographs of the mirror-polished magnet cross-sections
were taken by a polarization microscopy. The photograph was
processed to trace the interface between the magnet main body and
the plated film to produce a roughness curve, based on which the
10-point average roughness Rz was obtained in accordance with
JIS-B0601.
[0063] Each sample was also evaluated for its corrosion resistance
by a salt spray test in which the sample was immersed in a 5%
aqueous NaCl solution kept at 35.degree. C. for 240 hours. The
results are given in Table 1, where the samples were evaluated
according to the following standards:
.smallcircle.: Nothing abnormal detected .DELTA.: Partly rusted X:
Generally rusted
TABLE-US-00001 TABLE 1 Size of Grain size of 10-point milled
sintered average Adhesion Salt spray Salt spray powders body, D50
roughness Rz strength test results test results No. (.mu.m) (.mu.m)
(.mu.m) Rz/D50 (N/m) (240 hours) (720 hours) 1* 5.6 7.3 1.1 0.15 95
.largecircle. -- 2 5.7 7.4 1.6 0.22 134 .largecircle. .largecircle.
3 5.9 7.6 3.6 0.47 149 .largecircle. .largecircle. 4 5.7 7.4 5.9
0.80 165 .largecircle. .largecircle. 5 10.7 13.6 11 0.81 170
.largecircle. .largecircle. 6 3.2 4.3 3.5 0.81 160 .largecircle.
.largecircle. 7 3.3 4.4 6.5 1.48 195 .largecircle. .DELTA. 8 5.6
7.2 15.7 2.18 203 .largecircle. .DELTA. 9 4.0 5.2 15.1 2.90 210
.largecircle. .DELTA. 10 3.1 4.1 14.8 3.61 215 .largecircle.
.DELTA. 11 2.3 3.1 17.6 5.68 230 .largecircle. .DELTA. 12 2.4 3.3
25.4 7.70 238 .DELTA. -- 13 2.4 3.2 30.2 9.44 236 .DELTA. -- 14*
2.4 3.3 40.5 12.27 235 X -- 15* 3.2 4.2 69.9 16.64 240 X --
*Comparative Examples
[0064] As shown in Table 1 and FIG. 1, there is a strong
correlation between Rz/D50 ratio and adhesion strength; each of
Sample nos. 2 to 15 having an Rz/D50 ratio of 0.20 or more has an
adhesion strength of 100 N/m or more. An adhesion strength of 150
N/m or more can be secured at the ratio above 0.60.
[0065] It should be noted, however, that sintered magnet corrosion
resistance starts to gradually decrease as the ratio increases
beyond 6.00, and that it is generally rusted at the ratio above
10.00, as shown in Table 1. It is also noted, as shown in FIG. 1,
that the effect of improving adhesion strength increases to a
limited extent as the Rz/D50 ratio increases beyond 6.00. It can be
concluded, based on these results, that the preferable Rz/D50 ratio
is from 0.20 to 6.0, inclusive.
[0066] Sample nos. 2 to 11 were immersed in a 5% aqueous NaCl
solution for additional 480 hours (total immersion time of 720
hours) to visually observe the changes of sintered magnet surfaces,
in order to confirm the more preferable Rz/D50 range. Sample nos. 2
to 6 showed no change, whereas Sample nos. 7 to 11 were partly
rusted, as shown in Table 1. Therefore, it was found that the
sintered magnet has a still higher corrosion resistance when the
Rz/D50 ratio is kept at 0.20 to 1.00.
[0067] Comparing the results of Sample no. 6 with those of Sample
no. 10 having a similar D50 level, the latter was coated with the
plated film less uniformly than the former having a 10-point
average roughness of 3.5 .mu.m, and partly rusted at the thinner
film portions. The difference between them in formed plated film
condition comes from difference in their 10-point average
roughnesses. It is considered that Sample no. 10 is less
corrosion-resistant, in spite of being coated with the film of
higher adhesion strength, than Sample no. 6 because of its higher
10-point average roughness of about 15.0 .mu.m. Sample nos. 8, 9
and 11 having a 10-point average roughness of 15.0 .mu.m or more
were on a level with Sample no. 10 in corrosion resistance. It is
therefore concluded that keeping a 10-point average roughness of
13.0 .mu.m or less, preferably 10.0 .mu.m or less, is effective for
securing high corrosion resistance.
Example 2
[0068] A total of 9 types of samples similar to those prepared in
Example 1 were subjected to a high temperature/high humidity test,
where each was kept at 80.degree. C. and relative humidity 90% for
480 hours, to observe rusted conditions on the surface. The results
are given in Table 2, where the samples were evaluated according to
the following standards:
.smallcircle.: Nothing abnormal detected .DELTA.: Partly rusted X:
Generally rusted
TABLE-US-00002 TABLE 2 High Size of Grain size of 10-point
temperature/ milled sintered average Adhesion high powders body,
D50 roughness Rz strength humidity No. (.mu.m) (.mu.m) (.mu.m)
Rz/D50 (N/m) test results Br(G) 16 5.7 7.4 1.6 0.22 134
.largecircle. 12755 17 5.9 7.6 3.6 0.47 149 .largecircle. 12773 18
5.7 7.4 5.9 0.80 165 .largecircle. 12759 19 10.7 13.6 11 0.81 170
.largecircle. 12783 20 3.2 4.3 3.5 0.81 160 .largecircle. 12749 21
3.3 4.4 6.5 1.48 195 .largecircle. 12764 22 3.1 4.1 14.8 3.61 215
.DELTA. 12735 23* 2.4 3.3 40.5 12.27 235 X 12728 24* 3.2 4.2 69.9
16.64 240 X 12736 *Comparative Examples
[0069] As shown in Table 2, Sample nos. 23 and 24 having an Rz/D50
ratio above 10.00 were generally rusted. By contrast, Sample nos.
16 to 22 having an Rz/D50 ratio in a range from 0.20 to 6.00 were
rusted partly or showed no change on the sintered magnet
surfaces.
[0070] Sample nos. 16 to 24 were measured for residual magnetic
flux density (Br) using a B-H tracer. As shown in Table 2, Sample
nos. 16 to 22 showing good high temperature/high humidity test
results had a higher residual magnetic flux density (Br) than
Sample nos. 23 and 24. In particular, Sample nos. 18 to 21 having
an Rz/D50 ratio in a range from 0.50 to 1.50 had an adhesion
strength of 150 N/m or more and, at the same time, residual
magnetic flux density (Br) of 12,740 G or more. Sample nos. 18, 19
and 20 had a similar Rz/D50 ratio, although D50 and Rz levels of
one sample were much different from those of another. These results
confirm that it is important to control the Rz/D50 ratio in order
to secure high corrosion resistance and magnetic properties
simultaneously.
* * * * *